A SAW device produces a delayed echo when interrogated by an RF pulse (i.e., an interrogating signal, or incident signal) from a radio frequency (RF) transmitter. The RF pulse is also referred to as a chip or RF chip. The echo response or echo signal is produced passively, that is, similar to a light-reflecting mirror. However, like a distorting carnival mirror, the echo signal carries a signature of the reflecting device as well as a signature of the incident signal. The echo signal is re-transmitted from the SAW device to an RF receiver without the aid of active components. At the RF receiver the echo signal is analyzed and identified.
The characteristics of the reflected signal that are imparted by the SAW device is the focal point of the present invention.
FIG. 1 depicts a prior art SAW device 10. An interrogating or illuminating wave comprises a radio frequency (RF) signal pulse 12 transmitted by an RF transceiver 14, which is referred to in this document as a “SAW interrogator.” The transmitted signal travels through space to each SAW device within the field of view of the SAW interrogator. Only one SAW device 10 is illustrated in FIG. 1. The interrogating signal is received by an antenna 18 connected to an interdigital transducer (IDT) 20 disposed on a piezoelectric substrate 24. The IDT 20 launches an incident surface acoustic wave (SAW) 28 onto the piezoelectric substrate 24 in response to the received interrogation signal.
The SAW 28 propagates along the substrate 24 and is received at a reflector array 30 also disposed on the piezoelectric substrate 24. In applications where several SAW devices are within range of the RF transceiver each provides a reflected signal back to the RF transceiver. The reflector array 30 of each SAW device in a system of SAW devices (where the SAW device 10 is one SAW device within that system) comprises a unique pattern of metal electrodes 34 that impart a unique impulse response to the incident SAW 28. The impulse response of the reflector array 30 is imparted to the incident SAW 28 as it launches a reflected SAW 34 back to the IDT 20. The IDT 20 then converts the acoustic wave to an electrical signal that is radiated from the antenna 18 back to the RF transceiver 14 for extraction of the desired information in the reflected signal.
Let the mathematical function for the incident or interrogation SAW be defined as s(t), and a mathematical function for the impulse response function of the reflector array 30 be defined as h(t). Then the RF echo signal or the reflected SAW 34 signal, represented as y(t), received back at the IDT 20 and transmitted from the SAW device antenna 18 is computed as a conventional linear system output:Echo signal=y(t)=s(t)*h(t)                (where * is the convolution operator, not multiplication)        
The most basic application of such SAW devices is to simply identify (ID) an object to which the SAW device has been affixed. These devices are therefore often referred to as “tags” or “RFID tags.” SAW devices are also employed to remotely sense changes in the substrate (and thus changes to the SAW device elements disposed on the substrate) such as changes in temperature and applied stresses. These applications are beyond the scope of the current patent application. To clarify, these SAW devices are not representative of the current large annual market in active RFID tags. The SAW devices for such applications have not yet been widely marketed.
Until recently the described wireless-passive SAW devices could only be detected within a few feet of the SAW interrogator, since the RF signal returned by the devices is so weak (i.e., signal losses in the SAW device are quite high). However, due to recent improvements in the SAW devices and in the SAW interrogator electronics, SAW devices can now be interrogated at distances over about 20 ft. The number of SAW sensors or SAW tags that can operate within the detection volume grows as a third power of the detection range. As the detection range increases the number of reflected signals or echoes (one from each SAW device) increases and interference between these return signals or echoes also increases. There is therefore a growing interest in avoiding mutual interference between SAW devices.
A similar device is described by William R. Holland in U.S. Pat. No. 4,746,830. But the present invention offers improvements to Holland's device by adding amplitude weighting and specifying the spacing, Δf, of center frequencies of the SAW reflectors, such that the echoes produced from multiple sensors cause minimal mutual interference.